why do we see the 7 colors when light passes thru a prism but not thru a glass slab?
In the prism the sides through which a ray of light enters and leaves are inclined at an angle.
Inside the prism a white light is split up into its component colors and when they emerge out each ray is deviated at angle to the original direction of [white] ray.
Red is having a lesser deviation than the extreme violet ray.
In the case of slab, the sides are parallel and hence after spitted into component colors with in the slab and emerge out of the other side in a direction parallel to the original direction of [white] ray.
Considering a single white ray incident, the emergent rays are spitted into some distances; the red is nearer to the direction of original white ray and violet some what far away from the original direction.
However one cannot have a single ray in practice.
Hence when a red ray emerges out of the slab, in this direction all other components also emerge due to the other incident rays in contrast to prism. Hence again white ray is produced.
A drawing of rays could explain this easily.
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Refraction Of Light Through a Glass Slab
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